1. Field of the Invention
This invention relates generally to a surround for use in a plasma etching process and, more particularly, to a noncorrosive and noncontaminating magnesium surround for use in a plasma assisted chemical etch process.
2. Discussion of the Related Art
Plasma assisted chemical etch (PACE) is a known etching process by which a plasma generates chemically reactive species from a gas, such as a fluorine containing gas (e.g., CF.sub.4, SF.sub.6). A well known application of plasma etching is etching patterns into semiconductor device wafers in which a photo-lithographically defined pattern acts as a resist to the plasma etch. Another application is to modify a substrate by moving a small confined plasma over the surface of the substrate. Other applications include silicon-on-insulator (SOI) thinning, thickness variations of a substrate such as flattening of silicon wafers, and correcting the figure of an optical surface. For an example of SOI thinning, see U.S. patent application Ser. No. 07/696,897 filed May 7, 1991, U.S. Pat. No. 5,254,830, and for an example of alteration of an optical surface see U.S. Pat. No. 4,668,366 issued to Zarowin.
During the PACE process, an article being modified is secured to a platform within a vacuum chamber associated with a plasma reactor, and a plasma tool is positioned relative to the article. A feed gas (e.g., CF.sub.4, SF.sub.6) is introduced by the plasma tool into the plasma reactor chamber where the plasma generates chemically reactive species from the feed gas. The chemically reactive species interact with the substrate surface to form volatile by-products which etch the surface of the article. The chemical interaction rate may be increased by a flow of plasma generated excited species, low energy ions and excited neutral species to the surface.
A plasma etching system for modifying the substrate by the PACE process typically has three major components. These components include a device to measure the thickness profile of the article which is to be modified, computational hardware and software to provide a plasma tool velocity/position map based on the measured starting layer thickness profile, the desired thickness profile, and the plasma etch parameters which are to be used, and material removal hardware to execute the plasma tool motion and make the layer profile modifications.
When making a correction to the surface of the article by the PACE process, the plasma tool is scanned over the surface in relation to a dwell-time map that is calculated from a map of the corrections to be made and the plasma tool removal characteristics. To make an accurate correction of the article being modified, the plasma tool must be scanned over the edge of the article onto what is referred to as the surround. As is apparent from the name, the surround is the area surrounding the article being modified. The plasma tool motion as it moves off the article and onto the surround is given by the dwell-time map. The surround size should extend beyond the article being corrected by an amount that allows the full area of the plasma tool to be moved off of the article and onto the surround. In other words, it is necessary that the plasma tool be moved off of the article and onto the surround by such a degree that the plasma tool no longer causes material to be removed from the article. The above referenced patent application and patent provide a more detailed discussion of this process.
As is clear from the discussion above, the surround is subjected to the same etching removal process as the article being modified. Therefore, in applications in which repeated corrections are to be made using the same surround, erosion of the surround becomes a problem in that the surround may be etched along with the article. Further, residual byproducts of long term exposure to the PACE environment, as would occur in a production application, are deposited in the plasma reactor region. This deposition will cause particles on the article surface to act as micromasks to the plasma removal leaving corresponding microscopic bumps in the final surface of the article.
To reduce the effects of contamination, it has been known in the art to make the surround out of the same material as the article being modified. Consequently, the surround is etched at substantially the same rate as the article being modified during the PACE process. Therefore it becomes necessary to replace the surround at regular intervals. Since the PACE process is a vacuum process, replacement of the surround in a high volume production application in which the plasma apparatus is kept under vacuum results in a significant added expense and complexity to the process. Therefore it becomes desirable to provide a surround which does not cause contamination to the article being modified, and further, does not significantly erode in the plasma environment.
What is needed then is a surround consisting of a material which is not significantly eroded during the PACE process, and which does not contaminate the plasma reactor area or the article being modified. It is therefore an object of the present invention to provide such a surround.